A large part of today's traffic load in wireless communication comes from users inside physical structures such as office buildings, shopping malls, cafés and restaurants, etc. Providing indoor users with good coverage, high bit-rate and spectrally efficient communication from base stations outside the physical structure is very challenging due to, for example, the penetration loss that occurs when the communication signals propagates through the walls of the building.
A well-known way of enhancing the indoor coverage is to deploy a wireless communication system intended for indoor usage comprising an indoor radio base station (RBS) connected to a distributed antenna system (DAS), wherein the antennas are located indoors and distributed close to the users. An example of a DAS is a so called leaky cable which essentially is coaxial cables with slots or gaps along its entire length which enable the cable to “leak” electromagnetic waves into its surroundings. A leaky cable can be used both for transmitting and for receiving electromagnetic waves, i.e. it allows for two-way communication. Leaky cables have traditionally been used in for example mines, elevator shafts, along railway tunnels, etc. but have become increasingly popular to deploy in office buildings, shopping malls and other large indoor complexes.
Multiple-input multiple-output (MIMO) communications are typically applicable indoors where high bit rates are sought after. However, leaky cables are relatively expensive and complicated to install due to their weight and stiffness, and in multiple-stream applications such as in MIMO communications several cables need to be installed more or less in parallel which complicates the installation and makes them even more expensive to use. Since a leaky cable leaks energy along its entire length, and since installations often requires quite long leaky cables, the signal-to-noise ratio (SNR) experienced by a user device located close to the end of the leaky cable is much less than if it was located at the beginning of the cable. This leads to a very skew capacity distribution along the leaky cable which is undesired. One way to combat this “capacity skewness” is to introduce multiple radio frequency amplifiers, or repeaters, along the leaky cable that may amplify the signal propagating through the cable. However, these amplifiers are not only expensive but also need power sources at each installation point which both complicates and increases the cost for the installation even more. Thus, finding a way to offer good indoor coverage with high bit-rate and spectrally efficient communication using leaky cables, which also is cost efficient and not too complex to install, is therefore highly sought for.